Caption: Conjugating E.coli bacteria. Coloured transmission electron micrograph (TEM) of Escherichia coli bacteria undergoing conjugation (coupling). E.coli are gram-negative rod-shaped bacteria found in the healthy human intestine. They may cause food poisoning. Here, some bacteria are conjugating, a process by which DNA genetic material is transferred from bacteria to bacteria. Thin hollow "hairs" called pili are seen joining the bacteria through which the DNA passes. In this way genes, such as for antibiotic resistance, can be passed on. This "sex" between bacteria was discovered in 1946. Magnification: x6000 at 35mm size.